The measurement of analyte concentration, in particular, hydrogen ion concentration or pH is important in a number of research, industrial, and manufacturing processes. For instance, the measurement of pH is routinely practiced in food and beverage, biofuel, biophamaceuticals, as well as in the treatment of water and waste.
Many conventional pH sensors use a potentiometric approach which involves the use of glass electrode to measure pH. The potentiometric approach suffers from several drawbacks. One limitation of potentiometric sensors is the need for constant calibration. Potentiometric pH electrodes, like batteries, tend to run down with time and use. As the potentiometric electrode ages, its glass membrane tends to change in resistance, which in turn will alter the electrode potential. For this reason, the glass electrodes require calibration on a regular basis. The need for constant recalibration to provide an accurate pH output significantly impedes industrial applications especially where constant in-line pH measurements are required. Recalibration is particularly cumbersome in a biotech environment where pH measurement is conducted in medium containing biological materials. Another significant drawback of conventional pH sensors is that the glass electrodes have internal solutions, which in some cases can leak out into the solution being measured. The glass electrodes can also become fouled by species in the measuring solution, e.g., proteins, causing the glass electrode to foul. ISFET devices have been developed which use a field effect transistor structure on a silicon surface to measure pH (Bergveld E m et al., IEEE Sensor Conference, Toronto, October 2003, 1-26). These devices also have limitations. Thus, there remains a considerable need for reliable and consistent analyte sensors, and in particular, pH sensors.